AND OF THE CAUDAL END OF THE SPINAL CORD. 171 



tive vertebrae. In the younger specimens it emerges at the thirty-fifth scleromere, while 

 in this one it emerges at about the thirty-seventh and from thence runs ventral to the 

 spinal cord, touching the latter closely. The cloaca is situated at a level between the 

 thirty-third and thirty-fourth scleromeres. 



Embryo No. 485, 13 mm. Crown-Rump Length. 



This specimen is cut in transverse section and is therefore well suited for the study 

 of the spinal cord. There are 33 spinal ganglia, but at the thirty-third and thirty-second 

 complete nerve-fibers can not be made out. The thirty-third, in particular, comprises 

 such a small cell-group as to be hardly recognizable. There are 37 primitive vertebne 

 and a non-vertebrated tail portion 289 n long. As in the several specimens immediately 

 preceding it, the few caudal vertebrae are fused together, showing no distinct boundaries. 

 The non-vertebrated tail portion consists still of germinating mesenchymal cells, while 

 the more cranially situated scleromeres are gradually becoming converted into precar- 

 tilaginous tissue. 



Embryo No. 643, 13 m.m. Crown-Ru.mp Length. 



This specimen is cut in serial sagittal sections, but is, however, rather poorly preserved. 

 There are 37 primitive vertebrae, the last one being incomplete. Caudal to this is a long 

 non-vertebrated tail portion. There are 33 spinal ganglia, the last 2 having slender nerves 

 which, with the thirty-first, form the caudal nerves. These extend caudally along each 

 side of the tail. 



Embryo No. 940, 14 mm. Crown-Rump Length. 



This specimen is cut in transverse section, and it is therefore difficult to follow the 

 topography of some of the structures. I was able to count 36 primitive vertebrae, with a 

 remnant, and 33 spinal ganglia. The last two, especially the thirty-third, are small and 

 not provided with nerves. The twenty-ninth, thirtieth, and thirty-first spinal nerves extend 

 down along the sides of the cord to the level of the thirty-sixth vertebra. The central canal 

 of the spinal cord narrows at about the thirty-second vertebra, the portion caudal to 

 this being practically devoid of the mantle layer. 



Embryo No. 390, 15.5 mm. Crown-Rump Length. 



In embryo No. 390 the primitive vertebrae are differentiated into precartilaginous 

 tissue and between the vertebrae there is a small quantity of embryonic connective-tissue, 

 as indicated in figure 36. At this period the vertebral column consists of 35 precartilaginous 

 vertebrae; no additional segments can be recognized, although in the younger specimens 

 there were 38 somites and one very long mesodermic remnant. There is a long, irregular, 

 mesodermic cell-strand at the caudal end of the thirty-fifth segment, which, with the 

 spinal cord, extends to the end of the tail. Between the two lies the caudal end of the 

 chorda dorsalis, which, however, does not extend to the tip of the tail, as can be seen in 

 figures 8 and 36. About the thirty-fifth segment the chorda becomes embedded in the 

 substance of the vertebral column, the point at which it emerges being characterized by 

 a sharp curve in its course. 



We are here confronted with the following questions: WTiat was the fate of the addi- 

 tional somites which could be so distinctly recognized in the earlier stages? And what 

 is the mesodermic cell-strand which extends from the last primitive vertebra? As to the 

 first, from the study of this material I am of the opinion that the last few vertebrae, which 

 have earlier developed from the sclerotomes, fuse together during the process of embryonic 

 development, thus forming the last vertebra in an embryo of this age. I am, however, 

 aware that the comparison of a series of embryos can not conclusively settle this question. 

 As concerns the cell-strand, it is my belief that it is formed of parts of the tissue which did 



